55 research outputs found
New Image Statistics for Detecting Disturbed Galaxy Morphologies at High Redshift
Testing theories of hierarchical structure formation requires estimating the
distribution of galaxy morphologies and its change with redshift. One aspect of
this investigation involves identifying galaxies with disturbed morphologies
(e.g., merging galaxies). This is often done by summarizing galaxy images
using, e.g., the CAS and Gini-M20 statistics of Conselice (2003) and Lotz et
al. (2004), respectively, and associating particular statistic values with
disturbance. We introduce three statistics that enhance detection of disturbed
morphologies at high-redshift (z ~ 2): the multi-mode (M), intensity (I), and
deviation (D) statistics. We show their effectiveness by training a
machine-learning classifier, random forest, using 1,639 galaxies observed in
the H band by the Hubble Space Telescope WFC3, galaxies that had been
previously classified by eye by the CANDELS collaboration (Grogin et al. 2011,
Koekemoer et al. 2011). We find that the MID statistics (and the A statistic of
Conselice 2003) are the most useful for identifying disturbed morphologies.
We also explore whether human annotators are useful for identifying disturbed
morphologies. We demonstrate that they show limited ability to detect
disturbance at high redshift, and that increasing their number beyond
approximately 10 does not provably yield better classification performance. We
propose a simulation-based model-fitting algorithm that mitigates these issues
by bypassing annotation.Comment: 15 pages, 14 figures, accepted for publication in MNRA
On the Evolution of the Velocity-Mass-Size Relations of Disk-Dominated Galaxies over the Past 10 Billion Years
We study the evolution of the scaling relations between maximum circular
velocity, stellar mass and optical half-light radius of star-forming
disk-dominated galaxies in the context of LCDM-based galaxy formation models.
Using data from the literature combined with new data from the DEEP2 and AEGIS
surveys we show that there is a consistent observational and theoretical
picture for the evolution of these scaling relations from z\sim 2 to z=0. The
evolution of the observed stellar scaling relations is weaker than that of the
virial scaling relations of dark matter haloes, which can be reproduced, both
qualitatively and quantitatively, with a simple, cosmologically-motivated model
for disk evolution inside growing NFW dark matter haloes. In this model optical
half-light radii are smaller, both at fixed stellar mass and maximum circular
velocity, at higher redshifts. This model also predicts that the scaling
relations between baryonic quantities evolve even more weakly than the
corresponding stellar relations. We emphasize, though, that this weak evolution
does not imply that individual galaxies evolve weakly. On the contrary,
individual galaxies grow strongly in mass, size and velocity, but in such a way
that they move largely along the scaling relations. Finally, recent
observations have claimed surprisingly large sizes for a number of star-forming
disk galaxies at z \sim 2, which has caused some authors to suggest that high
redshift disk galaxies have abnormally high spin parameters. However, we argue
that the disk scale lengths in question have been systematically overestimated
by a factor \sim 2, and that there is an offset of a factor \sim 1.4 between
H\alpha sizes and optical sizes. Taking these effects into account, there is no
indication that star forming galaxies at high redshifts (z\sim 2) have
abnormally high spin parameters.Comment: 19 pages, 10 figures, accepted to MNRAS, minor changes to previous
versio
Morphologies of z~0.7 AGN host galaxies in CANDELS : no trend of merger incidence with AGN luminosity
PS would like to acknowledge funding through grant ASI I/005/11/0. DKoo would like to acknowledge funding through grant NSF AST-0808133. SJ acknowledges financial support from the EC through an ERC grant StG-257720.The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z < 0.8 and have luminosities 41 ≲ log (LX [erg s−1]) ≲ 44.5. ‘Fake AGN’ are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ∼950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for <6 per cent of all AGN in our sample as well as <40 per cent of the highest luminosity AGN (log  (LX [erg s−1]) ∼ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible.PostprintPeer reviewe
Structural Evolution of Early-type Galaxies to z=2.5 in CANDELS
Projected axis ratio measurements of 880 early-type galaxies at redshifts
1<z<2.5 selected from CANDELS are used to reconstruct and model their intrinsic
shapes. The sample is selected on the basis of multiple rest-frame colors to
reflect low star-formation activity. We demonstrate that these galaxies as an
ensemble are dust-poor and transparent and therefore likely have smooth light
profiles, similar to visually classified early-type galaxies. Similar to their
present-day counterparts, the z>1 early-type galaxies show a variety of
intrinsic shapes; even at a fixed mass, the projected axis ratio distributions
cannot be explained by the random projection of a set of galaxies with very
similar intrinsic shapes. However, a two-population model for the intrinsic
shapes, consisting of a triaxial, fairly round population, combined with a flat
(c/a~0.3) oblate population, adequately describes the projected axis ratio
distributions of both present-day and z>1 early-type galaxies. We find that the
proportion of oblate versus triaxial galaxies depends both on the galaxies'
stellar mass, and - at a given mass - on redshift. For present-day and z<1
early-type galaxies the oblate fraction strongly depends on galaxy mass. At z>1
this trend is much weaker over the mass range explored here
(10^10<M*/M_sun<10^11), because the oblate fraction among massive (M*~10^11
M_sun) was much higher in the past: 0.59+-0.10 at z>1, compared to 0.20+-0.02
at z~0.1. In contrast, the oblate fraction among low-mass early-type galaxies
(log(M*/M_sun)1 to
0.72+-0.06 at z=0. [Abridged]Comment: accepted for publication in ApJ; 14 pages; 10 figures; 4 table
No More Active Galactic Nuclei in Clumpy Disks Than in Smooth Galaxies at z~2 in CANDELS / 3D-HST
We use CANDELS imaging, 3D-HST spectroscopy, and Chandra X-ray data to
investigate if active galactic nuclei (AGNs) are preferentially fueled by
violent disk instabilities funneling gas into galaxy centers at 1.3<z<2.4. We
select galaxies undergoing gravitational instabilities using the number of
clumps and degree of patchiness as proxies. The CANDELS visual classification
system is used to identify 44 clumpy disk galaxies, along with mass-matched
comparison samples of smooth and intermediate morphology galaxies. We note
that, despite being being mass-matched and having similar star formation rates,
the smoother galaxies tend to be smaller disks with more prominent bulges
compared to the clumpy galaxies. The lack of smooth extended disks is probably
a general feature of the z~2 galaxy population, and means we cannot directly
compare with the clumpy and smooth extended disks observed at lower redshift.
We find that z~2 clumpy galaxies have slightly enhanced AGN fractions selected
by integrated line ratios (in the mass-excitation method), but the spatially
resolved line ratios indicate this is likely due to extended phenomena rather
than nuclear AGNs. Meanwhile the X-ray data show that clumpy, smooth, and
intermediate galaxies have nearly indistinguishable AGN fractions derived from
both individual detections and stacked non-detections. The data demonstrate
that AGN fueling modes at z~1.85 - whether violent disk instabilities or
secular processes - are as efficient in smooth galaxies as they are in clumpy
galaxies.Comment: ApJ accepted. 17 pages, 17 figure
Geometry of Star-Forming Galaxies from SDSS, 3D-HST and CANDELS
We determine the intrinsic, 3-dimensional shape distribution of star-forming
galaxies at 0<z<2.5, as inferred from their observed projected axis ratios. In
the present-day universe star-forming galaxies of all masses 1e9 - 1e11 Msol
are predominantly thin, nearly oblate disks, in line with previous studies. We
now extend this to higher redshifts, and find that among massive galaxies (M* >
1e10 Msol) disks are the most common geometric shape at all z < 2. Lower-mass
galaxies at z>1 possess a broad range of geometric shapes: the fraction of
elongated (prolate) galaxies increases toward higher redshifts and lower
masses. Galaxies with stellar mass 1e9 Msol (1e10 Msol) are a mix of roughly
equal numbers of elongated and disk galaxies at z~1 (z~2). This suggests that
galaxies in this mass range do not yet have disks that are sustained over many
orbital periods, implying that galaxies with present-day stellar mass
comparable to that of the Milky Way typically first formed such sustained
stellar disks at redshift z~1.5-2. Combined with constraints on the evolution
of the star formation rate density and the distribution of star formation over
galaxies with different masses, our findings imply that, averaged over cosmic
time, the majority of stars formed in disks.Comment: Published in ApJ Letter
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure
Are Compton-Thick AGN the Missing Link Between Mergers and Black Hole Growth?
We examine the host morphologies of heavily obscured active galactic nuclei (AGNs) at z ~ 1 to test whether obscured super-massive black hole growth at this epoch is preferentially linked to galaxy mergers. Our sample consists of 154 obscured AGNs with N_H > 10^(23.5) cm^(-2) and z 1.5. Using visual classifications, we compare the morphologies of these AGNs to control samples of moderately obscured 10^(22) cm^(-2) < N_H < 10^(23.5)cm^(-2) and unobscured (N_H < 10^(22) cm^(-2)) AGN. These control AGNs have similar redshifts and intrinsic X-ray luminosities to our heavily obscured AGN. We find that heavily obscured AGNs are twice as likely to be hosted by late-type galaxies relative to unobscured AGNs (65.3_(-4.6)^(+4.1)%) versus 34.5_(-2.7)^(+2.9)%) and three times as likely to exhibit merger or interaction signatures (21.5_(-3.3)^(+4.2)%) versus 7.8_(-1.3)^(+1.9)%). The increased merger fraction is significant at the 3.8σ level. If we exclude all point sources and consider only extended hosts, we find that the correlation between the merger fraction and obscuration is still evident, although at a reduced statistical significance (2.5σ). The fact that we observe a different disk/spheroid fraction versus obscuration indicates that the viewing angle cannot be the only thing differentiating our three AGN samples, as a simple unification model would suggest. The increased fraction of disturbed morphologies with obscuration supports an evolutionary scenario, in which Compton-thick AGNs are a distinct phase of obscured super-massive black hole (SMBH) growth following a merger/interaction event. Our findings also suggest that some of the merger-triggered SMBH growth predicted by recent AGN fueling models may be hidden among the heavily obscured, Compton-thick population
Structural parameters of galaxies in CANDELS
We present global structural parameter measurements of 109,533 unique, H-F160W-selected objects from the CANDELS multi-cycle treasury program. Sersic model fits for these objects are produced with GALFIT in all available near-infrared filters (H-F160W, J(F125W) and, for a subset, Y-F105W). The parameters of the best-fitting Sersic models (total magnitude, half-light radius, Sersic index, axis ratio, and position angle) are made public, along with newly constructed point-spread functions for each field and filter. Random uncertainties in the measured parameters are estimated for each individual object based on a comparison between multiple, independent measurements of the same set of objects. To quantify systematic uncertainties, we create a mosaic with simulated galaxy images with a realistic distribution of input parameters and then process and analyze the mosaic in an identical manner as the real data. We find that accurate and precise measurements-to 10% or better-of all structural parameters can typically be obtained for galaxies with H-F160W < 23, with comparable fidelity for basic size and shape measurements for galaxies to H-F160W similar to 24.5
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